-LRB- CNN -RRB- -- Before two pirate leaders departed the captured yacht where they held four Americans earlier this month , a maritime source says they left instructions : Kill the hostages if we do not come back from negotiations .

U.S. officials later took the two negotiating pirates into custody -- a move that goes against standard negotiation practices , the maritime source said .

The four Americans were later killed , but it is not clear why .

Also not clear is when during the negotiations -- or why -- the Americans detained the two pirate negotiators .

The pirates ' detention goes against standard negotiating practices , as the pirates came in good faith to make a deal to hand over the hostages , said the maritime source , who was briefed on the incident and has connections to British intelligence officials .

The source asked not to be identified because of the sensitivity of the situation .

U.S. Central Command declined to comment on whether officials detained the pirates and said the FBI and the Justice Department have the lead in the case . The Justice Department had no comment Sunday . On Friday , the FBI said there was no comment due to the continuing investigation .

However , a U.S. government official told CNN the talks with the two pirate negotiators faltered when it was determined the men `` had no authority to negotiate anything . '' American officials then took the men into custody rather then let them return to the yacht , the U.S. official said .

`` It would be unfathomable to have put them back on the boat where they might have done harm to the hostages , '' the official said , disagreeing with a published report that the detention of two fellow pirates might have upset those still aboard the captured yacht -- named the Quest -- who were holding four Americans .

`` There was no change in mood , no escalation '' and gunfire broke out on the Quest more than eight hours later , the U.S. official said .

The official took issue with a New York Times report that said the FBI 's hostage negotiator aboard the USS Sterett decided the two Somalis were not serious about resolving the matter , which led U.S. officials to take the two men into custody and ask for a new representative from the pirates .

According to the the U.S. official , who was not authorized to discuss the Quest hijacking on the record , the issue was not whether the two pirates were serious about negotiating , but that they lacked the power to cut a deal .

The American side continued trying to negotiate via radio with those still aboard the yacht and made an offer , and were told by the suspects aboard the Quest they would sleep on the matter before providing a reply , the U.S. official said . The Navy also tried to contact Somalis on land who might be able to exert influence over the men holding the Quest .

The FBI hostage negotiator is part of a special team based at Quantico , Virginia , and had field experience , the U.S. official said . It was unclear if that experience included any previous negotiations with Somali pirates .

Americans Jean and Scott Adam , Phyllis Macay and Bob Riggle were found shot to death after U.S. forces boarded the Quest around 1 a.m. Tuesday , U.S. officials have said .

The 58-foot yacht was being shadowed at the time by the military after pirates took it over off the coast of Oman on February 18 .

U.S. forces had responded after a rocket-propelled grenade was fired at a U.S. Navy ship about 600 yards away -- and missed -- and the sound of gunfire could be heard on board the Quest , U.S. Navy Vice Admiral Mark Fox has said . The killings took place as negotiations involving the FBI were under way for the hostages ' release .

When Fox spoke last week , he said two pirates boarded a U.S. Navy ship Monday for talks . He told reporters he had no information on details of the negotiations or whether a ransom had been offered .

Two pirates were found dead on board the Quest , said Fox . In the process of clearing the vessel , U.S. forces killed two others , he said . Thirteen other pirates were captured and detained on the Quest , along with the other two already aboard the U.S. Navy ship .

Fox said authorities believe the pirates were trying to get the vessel and hostages to Somalia , or at least into Somali territorial waters .

Piracy has flourished recently off the coast of Somalia , which has not had an effective government for two decades .

Globally , more than 50 pirate attacks have already taken place in 2011 . As of February 15 -- the most recent statistic posted on the International Maritime Bureau 's website , pirates were holding 33 vessels and 712 hostages .

CNN 's Zain Verjee and Carol Cratty contributed to this report

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NEW : A second source says two pirates were detained by U.S. forces

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Four Americans were killed after pirates hijacked their vessel

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Negotiations for their release had been under way , involving the FBI

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Two pirates had boarded a U.S. Navy ship Monday for talks